Permanent-magnet synchronous motors (hereinafter simply referred to as “motors”) are known as high-efficiency motors in comparison with most-used conventional induction motors, because magnetic field is established by the permanent magnet which results in no need for excitation current and no current flows through the rotor which results in no secondary copper loss. While induction motors are conventionally used in electric railway cars, application of permanent-magnet synchronous motors to electric railway cars has been investigated in recent years to enhance efficiency thereof.
In electric power converters used in controllers of electric railway cars that drive and control the permanent-magnet synchronous motors, a stable operation is required in which the possibility of a halt in operation due to failure or of a breakage is reduced to the extreme extent, to ensure stable services of electric railway cars. In order to achieve such a requirement, how to protect the electric power controllers is a most important subject in applying permanent-magnet synchronous motors to electric railway cars. In other words, it is necessary to clarify anomalous phenomena that might possibly occur in the electric power converters and to take proper measures against these anomalies so that excessive operation halting of the power converters is eliminated and the power converters are kept unbroken.
An electric power converter is made up of a large amount of electric/electronic parts, for example, electronic parts such as sensors and a microprocessor, semiconductor parts such as switching elements, electrical conductive members such as cables and bus bars that connect these parts with each other, and insulation members. For this reason, a failure or a malfunction could potentially occur in a constituent element, i.e., in each of the parts making up the electric power converter, and, on this occasion, anomalous phenomena such as an over-current and an over-voltage occur in the circuits. In addition, a temporary over-current or over-voltage may sometimes occur in the circuits by supply-voltage fluctuation specific to electric railways or by various disturbances associated with vibration of a railway car and that from the rails.
Moreover, permanent-magnet synchronous motors, different from induction motors having been most used, always generate voltage during rotation by interaction with magnetic field of the built-in permanent magnets even without electric power externally supplied.
Generally, electric railway cars run with a plurality of cars being coupled into a train, and a plurality of electric power converters and motors are each distributedly mounted on the plurality of cars. Thus, even when, for example, an electric power converter among the plurality of electric power converters on the train happens to halt owing to a failure, the railway cars can continue to run by the other normal motors. On the other hand, the motor connected with the halted electric power converter is forced to rotate continuously by the wheels, to generate a voltage in proportion to the rotation speed thereof.
Accordingly, depending on types of failures occurring in the electric power converter during running of the railway cars, a current is continuously supplied to the failure part by the motor-generated voltage, so that the failure part might be further damaged or might causes heat build-up.
For such a case, a protective function is required for preventing the electric power converter from breaking, in such a manner that a system controller monitors signals from a voltage sensor, a current sensor, or thee like provided in the electric power converter and, when a monitored value, for example, exceeds a predetermined one or the like, the system controller determines that an anomalous phenomenon has occurred and turns off internal contactors and switching elements according to predetermined logic rules.
However, it is improper to turn off all the contactors and switching elements in the electric power converter when an anomalous phenomenon occurs, because it takes time to restart the electric power converter, which retards on-time service of the railway cars, and operation counts of the contactors and the like increases more than necessary, which accelerates wear of moving parts thereof. Moreover, damage of anomalous portions might increase if improper measures are taken. The electric power converter therefore needs to have a protective function that allows proper measures to be taken against various types of anomalous phenomena and failures occurring in the electric power converter in order to prevent the converter from being damage while avoiding excessive operation halting thereof.
A method is disclosed as a prior art in Patent Document 1, in which, supposing the case where a failure occurs in an inverter that drives and controls a permanent-magnet synchronous motor while the electric railway cars are running, contactors are provided for shutting off the connection between the inverter and the permanent-magnet synchronous motor so as not to cause a further damage of the inverter by the motor-generated electric power, whereby the inverter and the permanent-magnet synchronous motor are isolated from each other by the contactors when a failure of the inverter is detected.
Patent Document 1: Japanese Patent Application Laid-Open No. H08-182105.